The present invention relates to TiAl-base alloys with excellent oxidation resistance, and a method for producing the same.
TiAl-base alloys are lightweight and have excellent high-temperature strength and creep strength, and the industrialization for practical use of the alloys as lightweight, heat-resistant materials is being promoted. However, there are problems in that the alloys are greatly oxidized when heated at high temperatures in oxidizing atmospheres. Therefore, in order to use such TiAl-base alloys as heat-resistant materials, the improvement in the oxidation resistance of the alloys is indispensable. For this, additions of ternary elements or surface treatments of the alloys are under study.
For example, the additions of ternary elements, such as chromium (Cr), molybdenum (Mo), niobium (Nb), silicon (Si), tantalum (Ta), tungsten (W), etc., to TiAl-base alloys may improve the oxidation resistance of the alloys.
However, the alloys still cannot exhibit satisfactory oxidation resistance in air at high temperatures of 900xc2x0 C. or higher since their oxidation rate is high under those conditions.
On the other hand, examples of surface treatments of TiAl-base alloys to improve the oxidation resistance, include (1) heat treatment under a low partial pressure oxygen atmosphere, (2) aluminizing, (3) chromizing, etc. However, these are not always effective in view of the adhesion between the matrix and the film formed thereon and in view of the long-term stability of the film.
In order to solve these problems, xe2x80x9cTiAl-base intermetallic compound materials and a method for producing the samexe2x80x9d (Japanese Patent Application Laid-Open No. 5-78817) have been proposed, which are characterized in that an Mo and or W-rich layer having a thickness of 0.5 xcexcm or larger is formed on the surface of a TiAl-base intermetallic compound material in the direction of its depth, through sputtering with diffusion coating or through heating in the presence of Mo and/or W oxides optionally followed by diffusion coating. They say that the Mo or W-rich layer formed could promote the formation of an Al2O3 layer in the lateral direction of the alloy, thereby greatly improving the oxidation resistance of the alloy.
However, xe2x80x9cTiAl-base intermetallic compound materials and a method for producing the samexe2x80x9d, disclosed in Japanese Patent Application Laid-Open No. 5-78817 has problems in various points such as those mentioned below. In the TiAl-base intermetallic compound material of that type, much Ti exists in the vicinity of its surface. Therefore, the mere formation of such an Mo and/or W-rich layer on its surface cannot provide a protective film with satisfactory oxidation resistance. Accordingly, in oxidizing atmospheres in air, not only such a protective film of alumina but also TiO2 is formed. TiO2 thus formed grows toward the inside of the substrate, thereby greatly lowering the oxidation resistance of the alloy. For these reasons, it is impossible to form a protective film with satisfactory oxidation resistance on the alloy.
In addition, the disclosed technique requires the adhesion of Mo and/or W followed by diffusion coating to be effected at a temperature between 700xc2x0 C. and 1450xc2x0 C., which will unfavorably cause sintering and solidification of the metal powders. Such solidified metal powders will adhere onto the TiAl-base intermetallic compound material.
Moreover, when a TiAl-base intermetallic compound material is heated in the presence of Mo and/or W oxides in a closed vessel, the oxides will also be unfavorably sintered and solidified like the metal powders, causing the same problem in that the thus-solidified oxides will adhere onto the TiAl-base intermetallic compound material. The adhered oxides will react with the TiAl-base intermetallic compound material in oxidizing atmospheres to give products having a low melting point, whereby the oxidation resistance of the material will be rather lowered. In addition, the heat treatment in the closed vessel has problems in point of the producibility.
(Objects of the Invention)
An object of the present invention is to provide TiAl-base alloys with excellent oxidation resistance.
Another object of the invention is to provide a method for producing TiAl-base alloys with excellent oxidation resistance.
(Constitution of the Invention)
The method of the present invention for producing TiAl-base alloys with excellent oxidation resistance is characterized in that a TiAl-base alloy material comprising a TiAl-base alloy with an Al content of from 15 at. % to 55 at. % is heated in the presence of an oxide having a smaller negative value of standard free energy of formation than that of alumina.
(Operation of the Invention)
Though as yet not clarified completely, the mechanism of the method of the invention to produce TiAl-base alloys with excellent oxidation resistance may be considered as follows.
First, a TiAl-base alloy material comprising a TiAl-base alloy with an Al content of from 15 at. % to 55 at. % is prepared.
Next, this TiAl-base alloy material is heated in the presence of an oxide having a smaller negative value of standard free energy of formation than that of alumina.
As a result, a TiAl-base alloy is obtained with ease, which comprises a substrate of a TiAl-base alloy with an Al content of from 15 at. % to 55 at. % and a protective film-forming layer with excellent oxidation resistance as formed on the surface of the substrate.
The surface layer of the TiAl-base alloy comprises at least one substance of: an oxide of the element that constitutes the above-mentioned oxide having a smaller negative value of standard free energy of formation than that of alumina; a composite oxide mainly composed of the element or an oxide of the element; and Al2O3 (alumina). The TiAl-base alloy having the surface layer of that type forms a stable Al2O3 film, while inhibiting the formation of TiO2 on its surface, in high-temperature oxidizing atmospheres. Accordingly, the oxidation resistance of the TiAl-base alloy is significantly improved.
(Effects of the Invention)
According to the method of the present invention for producing TiAl-base alloys, it is easy to obtain TiAl-base alloys with excellent oxidation resistance.
FIG. 1 is an explanatory view showing the fluidized bed furnace as used in Example 5 to Example 8 of the present invention.